This invention relates to cryoablation systems for treating biological tissues, and more particularly, to cryoablation balloon catheters using refrigerants in the liquid state.
Cryoablation therapy involves application of extremely low temperature and complex cooling systems to suitably freeze the target biological tissues to be treated. Many of these systems use cryoprobes or catheters with a particular shape and size designed to contact a selected portion of the tissue without undesirably affecting any adjacent healthy tissue or organ. Extreme freezing is produced with some types of refrigerants that are introduced through the distal end of the cryoprobe. This part of the cryoprobe must be in direct thermal contact with the target biological tissue to be treated.
There are various known cryoablation systems including for example liquid nitrogen and nitrous oxide type systems. Liquid nitrogen has a very desirable low temperature of approximately −200° C., but when it is introduced into the distal freezing zone of the cryoprobe which is in thermal contact with surrounding warm biological tissues, its temperature increases above the boiling temperature (−196° C.) and it evaporates and expands several hundred-fold in volume at atmospheric pressure and rapidly absorbs heat from the distal end of the cryoprobe. This enormous increase in volume results in a “vapor lock” effect when the internal space of the cryoprobe gets “clogged” by the gaseous nitrogen. The associated heat exchanger systems within the cryoprobes are also not compatible with the desired miniature size of probe tips that need to be less than 3 mm in diameter. Additionally, in these systems the gaseous nitrogen is simply rejected directly to the atmosphere during use which produces a cloud of condensate upon exposure to the atmospheric moisture in the operating room and requires frequent refilling or replacement of the liquid nitrogen storage tank.
Nitrous oxide and argon systems typically achieve cooling by expansion of the pressurized gases through a Joule-Thomson expansion element such as a small orifice, throttle, or other type of flow constriction that are disposed at the end tip of the cryoprobe. For example, the typical nitrous oxide system pressurizes the gas to about 5 to 5.5 MPa to reach a temperature of no lower than about −85 to −65° C. at an outlet pressure of about 0.1 MPa. For argon, the temperature of about −160° C. at the same pressure of 0.1 MPa is achieved with an initial pressure of about 21 MPa. The nitrous oxide cooling system is not able to achieve the temperature and cooling power provided by liquid nitrogen systems. Nitrous oxide and Argon-based cooling systems have some advantages because the inlet of high pressure gas at room temperature, when it reaches the Joule-Thompson (JT) throttling component or other expansion device at the probe tip, precludes the need for thermal insulation of its inlet components. However, because of the insufficiently low operating temperature, combined with relatively high initial pressure, cryosurgical applications are strictly limited. Additionally, the Joule-Thomson system typically uses a heat exchanger to cool the incoming high pressure gas using the outgoing expanded gas in order to achieve the necessary drop in temperature by expanding compressed gas. The cold returning gas also requires insulation to avoid freezing nontarget tissues along the course of the cryoprobe from the active tip segment Although an argon system is capable of achieving a desirable cryoablation temperature, argon systems do not provide sufficient cooling power and require very high gas pressures. These limitations are very undesirable because the corresponding probe diameters are currently limited to approximately 1.5 mm OD to allow sufficient high-volume gas flow for JT cooling, which is larger than what is needed for a number of applications.
Another cryoablation system uses a fluid at a near critical or supercritical state. Such cryoablation systems are described in U.S. Pat. Nos. 7,083,612 and 7,273,479. These systems have some advantages over previous systems. The benefits arise from the fluid having a gas-like viscosity. Having operating conditions near the critical point of nitrogen enables the system to avoid the undesirable vapor lock described above while still providing good heat capacity. Additionally, such cryosystems can use small channel probes.
However, challenges arise from use of a near-critical cryogen in a cryoablation system. In particular, there is still a significant density change in nitrogen (about 8 times) once it is crossing its critical point—resulting in the need for long pre-cooling times of the instrument. The heat capacity is high only close to the critical point and the system is very inefficient at higher temperatures requiring long pre-cooling times. Additionally, the system does not warm up (or thaw) the cryoprobe efficiently. Additionally, near-critical cryogen systems require a custom cryogenic pump which is more difficult to create and service.
Still other types of cryosystems are described in the patent literature. U.S. Pat. Nos. 5,957,963; 6,161,543; 6,241,722; 6,767,346; 6,936,045 and International Patent Application No. PCT/US2008/084004, filed Nov. 19, 2008, describe malleable and flexible cryoprobes. Examples of patents describing cryoablation systems for supplying liquid nitrogen, nitrous oxide, argon, krypton, and other cryogens or different combinations thereof combined with Joule-Thomson effect include U.S. Pat. Nos. 5,520,682; 5,787,715; 5,956,958; 6074572; 6,530,234; and 6,981,382.
Various cryo-energy delivering balloon catheters have been described in the patent literature. U.S. Pat. No. 6,736,809, for example, is directed to a method for treating an aneurysm by cooling a target tissue region of the aneurysm to a temperature below temperature for a preselected time period. The method entails thickening, strengthening, or increasing the density of a blood vessel wall by cooling the blood vessel wall with a cryogenically cooled device. In particular, a device having a heat conductive cooling chamber is disposed proximate to the aneurysm site; and a cryogenic fluid coolant is directed to flow inside the chamber to create endothermic cooling relative to the aneurysm.
U.S. Pat. No. 6,283,959 is also directed to a cryo-energy delivery device. The device described in the '959 patent uses carbon dioxide (CO2) and has a metallic balloon surface with different patterns for greater thermal conductivity. The '959 patent describes use of a non-toxic fluid to fill the balloon such as CO2, or nitrous oxide (N2O), in case of balloon rupture. The '959 patent also describes use of evaporative and JT cooling aspects by injecting a predominant liquid mixture under pressure and allowing evaporation and gas expansion. In addition, these gases are generally functional within the engineering constraints of most balloons and catheters of less than 500 psi pressure. However, with CO2 and N2O having respective boiling points of −78.5° C. and −88.5° C., it is doubtful that the surface temperatures of a balloon in contact with a vessel wall inside a blood vessel can reach anything lower than approximately −10° C. It is therefore uncertain, or perhaps unlikely, that any of the desired “positive remodeling” needed to keep an artery open to its balloon-dilated extent would be possible since temperatures required to get this stent-like effect need to be less than −40° C.
In addition, if nerve ablation is desired for treating hypertension by ablating the renal nerve adjacent the renal artery, temperatures below −60° C. may be needed for long-term prevention of nerve regrowth and the lasting effects on blood pressure. Therefore, it is uncertain, if not unlikely, that the above described cryo-balloons can achieve the desired temperatures within a biological system because of the physical limitations necessary for evaporative or JT-based cryosystems.
The above mentioned '809 and '959 patents do not describe a design for the generation of sufficiently low temperatures to obtain the desired cryo-physiologic response. Insufficient generation of cold temperatures arise from the physical limitations of the cooling mechanisms, as well as the physical engineering limitations, proposed in the above mentioned patents.
An improved cryoablation balloon catheter that overcomes the above mentioned drawbacks is therefore desirable.
An improved cryoablation balloon catheter that achieves minimal temperatures of less than −40° C. within several millimeters of the balloon surface into adjacent tissue, or vessel wall, is desirable to achieve desired vascular effects from positive remodeling. This is desirable in treating, for example, aneurysms, and to treat hypertension by renal nerve ablation. A cryoablation balloon catheter design is thus desirable that achieves the necessary therapeutic temperatures within the engineering and anatomical constraints.